Reports of dead fish of all sizes and species floating down the English River began surfacing on July 14 when residents of Howick, Très-Saint-Sacrement, and Sainte-Martine started posting their observations online. As the number of dead fish continued to mount over the following two weeks, so too did the hypotheses as to what might be causing the alarming phenomenon.
The Société de conservation et d’aménagement des bassins versants de la zone Châteauguay (SCABRIC) issued a statement on July 26 referring to comments made on social media linking the die-off to the presence of pesticides in the water, saying this type of assertion was close to disinformation.
Biologist Geneviève Audet, who is the assistant director of the SCABRIC, noted several possible explanations for a mass mortality event including the impact of warmer temperatures. “In hot weather, especially just before dawn in shallow environments, aquatic plants consume more oxygen than they emit, which can cause fish to suffocate,” she explains, noting temperatures above heat-tolerance thresholds for fish can also result in death. Witnesses have pointed out,however, that water levels in the English River were not low and that temperatures were not exceptionally hot when the dead fish began to appear.
Audet explains that the use of pesticide sprays in the region means small concentrations of chemicals can wash into streams with rain in the spring when pesticide use is more prevalent. She admits large quantities of pesticides, when absorbed directly, can result in fish death, but suggests pesticides in small quantities are more likely to cause deformations or reproductive disorders.
Finally, the addition of organic matter, from outdated or non-compliant sanitary installations or fertilizers spread on residential properties or farmland, favours the growth of aquatic plants, especially in warm temperatures. “When nutrients are available in large quantities and when it is warm and light is available, aquatic plants have no limit to their growth,” says Audet, referring to the phenomenon known as eutrophication. This can deplete the oxygen concentration in the water available to fish.
The SCABRIC points out mass die-offs have occurred in the past, most notably in Missisquoi Bay in 2020 and in the Yamaska River near Saint-Hyacinthe in 2016. And while the exact cause of this mass mortality event remains unknown, the Ministry for the Environment and the Fight Against Climate Change has been informed and is investigating. (SR)